1. Field of the Invention
The present invention relates to a cable connection structure and an electronic apparatus having the same, and more particularly to a cable connection structure which can be suitably applied to cables for signal transmission between circuit boards in an electronic apparatus, and to an electronic apparatus to which such a cable structure is applied.
2. Description of the Related Art
In recent years, with the development of electronic apparatus exhibiting higher speed and better performance, the influence of radiated electromagnetic waves from one electronic apparatus upon other electronic apparatuses has become a serious problem. The influence of radiated electromagnetic waves from one electronic apparatus upon other electronic apparatuses is called EMI (Electro Magnetic Interference), and causes interference to reception of radio devices and communication devices in particular, or malfunctioning of electronic apparatuses. To cope with this problem, various countries have prescribed that the frequency range of 30 MHz to 1 GHz or the range of 30 MHz to 2 GHz is a frequency range that can cause a problem of radiated electromagnetic waves, and control of the amount of radiation of electromagnetic waves in the above frequency range is now strictly carried out. Manufacturers of electronic apparatuses have to design and manufacture their products so as to comply with these regulations.
Radiation sources of radiated electromagnetic waves from inside electronic apparatuses include circuit boards, cables connected to circuit boards, etc. In particular, cables that connect circuit boards for signal transmission have become more likely to cause a problem as radiation sources of radiated electromagnetic waves as the signal transmission rate has become higher in recent years. Conventionally, to suppress such radiated electromagnetic waves, shielded cables covered with shielding conductors have been used, or a ferrite core has been mounted on a cable to suppress such radiated electromagnetic waves.
FIG. 35 is a view showing a conventional example of a cable connection structure for suppression of radiated electromagnetic waves. FIG. 35 shows an example of a case in which two circuit boards are connected to each other by a cable for signal transmission in an electronic apparatus. The circuit boards 1401, 1402 are conductively connected via metal spacers 1404 to a metal casing 1403 in the electronic apparatus. The circuit boards 1401, 1402 are connected to each other via a cable 1405 so that signals are transmitted via the cable 1405.
To suppress electromagnetic wave radiated from this cable 1405, a shielded cable has been conventionally used as the cable 1405. Alternatively, a ferrite core 1406 may be mounted on the cable 1405 as a part for suppressing radiated electromagnetic waves. As other methods for suppressing radiated electromagnetic waves, it can be envisaged that a magnetic material is mounted on a shielding conductor of a shielded cable as disclosed in Japanese Laid-Open Patent Publication (Kokai) No. 06-176823, a resistor is inserted in a shielding conductor of a shielded cable as disclosed in Japanese Laid-Open Patent Publication (Kokai) No. 09-22624, or a dielectric material is mounted a cable conductor of a shielded cable as disclosed in Japanese Laid-Open Patent Publication (Kokai) No. 10-27987.
However, there has been the following problems with the above described prior art. That is, in the case of signal transmission via a cable between circuit boards in an electronic apparatus, a resonance may arise due to the connection structure of the cable and circuit boards, and at the resonance frequency, radiated electromagnetic waves are efficiently radiated from the cable and the circuit boards. This resonance arises even if the cable is a shielded cable. Even if a shielded cable is used, radiated electromagnetic waves are radiated with a degree of intensity exceeding a predetermined regulated value. The electromagnetic wave radiation from the shielded cable is caused by a high frequency current flowing in the outer skin of the shielding conductor of the shielded cable. At the resonance frequency, in particular, the outer skin of the shielding conductor has a low impedance, and a large high frequency current flows in the outer skin, resulting in intense electromagnetic waves being generated.
The above-mentioned high frequency current may be suppressed, for example, by mounting a ferrite core on the shielded cable as shown in FIG. 35 to thereby increase the impedance of the outer skin of the shielding conductor. This effect of a ferrite core, however, is only effective in a low frequency region of about 300 MHz or below. Thus, in the frequency region exceeding 300 MHz up to 1 GHz or 2 GHz, in which the amount of electromagnetic wave radiation is regulated, a large high frequency current flows in the outer skin of the shielding conductor, and hence radiated electromagnetic waves cannot be suppressed.
As regards the magnetic material disclosed in Japanese Laid-Open Patent Publication (Kokai) No. 06-176823, the effect of the magnetic material is small in the high frequency region exceeding 300 MHz in the same manner as is the case with the above described ferrite core. As regards the resistor disclosed in Japanese Laid-Open Patent Publication (Kokai) No. 09-22624, although the insertion of the resistor into the shielding conductor has an effect in the whole frequency range, the effect is small, and is insufficient to suppress radiated electromagnetic waves in the frequency range where intense radiated electromagnetic waves are generated. Increasing the resistance value of the resistor may enhance the suppression effect, but it also reduces the return current flowing in the inner skin of the shielding conductor, and is therefore impracticable. As regards the dielectric material disclosed in Japanese Laid-Open Patent Publication (Kokai) No. 10-27987, the lower the frequency of the radiated electromagnetic waves, the longer the part has to be formed, which leads to a problem of limited mounting space.
According to the above described methods using a ferrite core, a resistor, and a dielectric material, a new component has to be prepared for the reduction of radiated electromagnetic waves, resulting in increased costs of the electronic apparatuses.
It is an object of the present invention to provide a cable connection structure having circuit boards of an electronic apparatus connected to each other by a shielded cable for transmission of signals, which is capable of effectively suppressing radiated electromagnetic waves due to resonance from the shielded cable and the circuit boards in an inexpensive and simple manner, and an electronic apparatus having the cable connection structure.
To attain the above object, a first aspect of the present invention provides a cable connection structure for use in an electronic apparatus, comprising at least two circuit boards, at least two connectors provided on the circuit boards, respectively, and a shielded cable connecting the circuit boards to each other via the connectors, the shielded cable having a shielding conductor as a covering, wherein the shielding conductor is conductively connected to the metal casing at a position spaced from a position where the shielding conductor and one of the connectors are connected to each other by a distance which is less than a value obtained by multiplying xc2xd of a wavelength at an upper limit frequency of relevant radiated electromagnetic waves emitted from the shielded cable or xc2xd of a wavelength at a regulated upper limit frequency of radiated electromagnetic waves by a wavelength reduction ratio at the shielding conductor.
To attain the above object, the first aspect of the present invention further provides a cable connection structure for use in an electronic apparatus, comprising at least two circuit boards, at least two connectors provided on the circuit boards, respectively, and a shielded cable connecting the circuit boards to each other via the connectors, the shielded cable having a shielding conductor as a covering, wherein the shielding conductor is conductively connected to the metal casing at a position spaced from a position where the shielding conductor and one of the connectors are connected to each other by a distance which is set to a value obtained by multiplying {fraction (1/32)} to {fraction (7/16)} of a wavelength at an upper limit frequency of relevant radiated electromagnetic waves emitted from the shielded cable or {fraction (1/32)} to {fraction (7/16)} of a wavelength at a regulated upper limit frequency of radiated electromagnetic waves by a wavelength reduction ratio at the shielding conductor.
To attain the above object, the first aspect of the present invention also provides a cable connection structure for use in an electronic apparatus, comprising at least two circuit boards, at least two connectors provided on the circuit boards, respectively, and a shielded cable connecting the circuit boards to each other via the connectors, the shielded cable having a shielding conductor as a covering, wherein the shielding conductor is conductively connected to the metal casing at a position spaced from a position where the shielding conductor and one of the connectors are connected to each other by a distance within a range of 1 cm to 13 cm.
Preferably, the cable connection structure according to the present invention further comprises adjusting means for adjusting a position of the shielded cable within a range which is set to a value obtained by multiplying {fraction (1/32)} to {fraction (7/16)} of the wavelength at the upper limit frequency of relevant radiated electromagnetic waves emitted from the shielded cable or {fraction (1/32)} to {fraction (7/16)} of the wavelength at a regulated upper limit frequency of radiated electromagnetic waves by the wavelength reduction ratio at the shielding conductor from the position where the shielding conductor and the one of the connectors are connected to each other.
In a preferred embodiment of the present invention, the circuit boards comprise a driver board that transmits signals, and a receiver board that receives signals, and wherein the shielding conductor is conductively connected to the metal casing at a position spaced from the position where the shielding conductor and one of the connectors which is provided on the driver board are connected to each other by a distance which is set to a value obtained by multiplying {fraction (1/32)} to {fraction (7/16)} of a wavelength at an upper limit frequency of relevant radiated electromagnetic waves emitted from the shielded cable or {fraction (1/32)} to {fraction (7/16)} of a wavelength at a regulated upper limit frequency of radiated electromagnetic waves by a wavelength reduction ratio at the shielding conductor.
More preferably, a characteristic impedance of a first portion of the shielding conductor relative to the metal casing from the position where the shielding conductor is conductively connected to the metal casing to the position where the shielding conductor and the one of the connectors provided on the driver board are connected to each other is greater than a characteristic impedance of a second portion of the shielding conductor relative to the metal casing from the position where the shielding conductor is conductively connected to the metal casing to a position where the shielding conductor is connected to another one of the connectors which is provided on the receiver board.
Also preferably, a distance between the shielding conductor and the metal casing at at least part of an area from the position where the shielding conductor is conductively connected to the metal casing to the position where the shielding conductor and the one of the connectors provided on the driver board are connected to each other is greater than a distance between the shielding conductor and the metal casing at other areas than the at least part of the area.
Preferably, the cable connection structure according to the present invention further comprises a resistance component serially connected between the shielding conductor and the metal casing at the position where the shielding conductor is conductively connected to the metal casing.
In a preferred embodiment of the present invention, the shielded cable comprises a multiconductor shielded cable for simultaneously transmitting a plurality of signals.
More preferably, the cable connection structure according to the present invention further comprises a ferrite core mounted on the shielded cable.
Also preferably, the cable connection structure according to the present invention further comprises a conductive connection member that conductively connects the shielding conductor of the shielded cable and the metal casing to each other, and a ferrite core mounted on the conductive connection member.
The shielded cable may have a circular cross section, or has a sheet-like shape.
Preferably, the shielded cable having a sheet-like shape includes a signal line, and the shielding conductor of the shielded cable entirely wraps the signal line.
Alternatively, the shielding conductor of the shielded cable having a sheet-like shape is provided only on one side of the shielded cable.
The cable connection structure according to the first aspect of the present invention employs a cable connection construction that circuit boards of an electronic apparatus are connected with a shielded cable for transmission of signals, and is designed such that generation of electromagnetic waves due to resonance from the shielded cable or the circuit boards can be effectively suppressed in an inexpensive and simple manner.
To attain the above object, a second aspect of the present invention provides an electronic apparatus having a metal casing, and a cable connection structure comprising at least two circuit boards, at least two connectors provided on the circuit boards, respectively, and a shielded cable connecting the circuit boards to each other via the connectors, the shielded cable having a shielding conductor as a covering, wherein the shielding conductor is conductively connected to the metal casing at a position spaced from a position where the shielding conductor and one of the connectors are connected to each other by a distance which is less than a value obtained by multiplying xc2xd of a wavelength at an upper limit frequency of relevant radiated electromagnetic waves emitted from the shielded cable or xc2xd of a wavelength at a regulated upper limit frequency of radiated electromagnetic waves by a wavelength reduction ratio at the shielding conductor.
To attain the above object, the second aspect of the present invention further provides an electronic apparatus having a metal casing, and a cable connection structure comprising at least two circuit boards, at least two connectors provided on the circuit boards, respectively, and a shielded cable connecting the circuit boards to each other via the connectors, the shielded cable having a shielding conductor as a covering, wherein the shielding conductor is conductively connected to the metal casing at a position spaced from a position where the shielding conductor and one of the connectors are connected to each other by a distance which is set to a value obtained by multiplying {fraction (1/32)} to {fraction (7/16)} of a wavelength at an upper limit frequency of relevant radiated electromagnetic waves emitted from the shielded cable or {fraction (1/32)} to {fraction (7/16)} of a wavelength at a regulated upper limit frequency of radiated electromagnetic waves by a wavelength reduction ratio at the shielding conductor.
To attain the above object, the second aspect of the present invention also provides an electronic apparatus having a metal casing, and a cable connection structure comprising at least two circuit boards, at least two connectors provided on the circuit boards, respectively, and a shielded cable connecting the circuit boards to each other via the connectors, the shielded cable having a shielding conductor as a covering, wherein the shielding conductor is conductively connected to the metal casing at a position spaced from a position where the shielding conductor and one of the connectors are connected to each other by a distance within a range of 1 cm to 13 cm.
The electronic apparatus according to the second aspect of the present invention employs a cable connection construction that circuit boards of an electronic apparatus are connected to each other by a shielded cable for transmission of signals, and is designed such that generation of electromagnetic waves due to resonance from the shielded cable or the circuit boards can be effectively suppressed in an inexpensive and simple manner.
The above and other objects of the invention will become more apparent from the following detailed description taken in conjunction with the accompanying drawings.